1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus and an electrostatic recording type image forming apparatus such as copying machines and printers, and more particularly, to an image forming apparatus having one photosensitive member drum and an intermediate transfer belt.
2. Description of the Related Art
Conventionally, a single-drum intermediate transfer belt type color image forming apparatus (apparatus for forming a color image using one photosensitive member drum and an intermediate transfer belt) has a problem that color deviation is generated due to variation in load such as approaching and separating motions of a transfer cleaner and a secondary transfer roller.
A mechanism of generation of the color deviation will be explained. FIG. 7(a) is a diagram used for explaining an essential portion of a conventional image forming apparatus. The image forming apparatus shown in FIG. 7(a) is a single-drum intermediate transfer belt type color image forming apparatus. In this structure, toner images of various colors formed on a photosensitive member drum 101 are transferred to an intermediate transfer belt 102 by a primary transfer unit 102a in succession and with this operation, the toner images are superposed and transferred on a surface of the intermediate transfer belt 102, thereby forming a color toner image. A recording material P is conveyed in synchronization with the toner images on the intermediate transfer belt 102, and toner images are collectively transferred on the recording material P by a secondary transfer unit 102b. 
A secondary transfer roller 103 which is secondary transfer means can approach and separate from the intermediate transfer belt 102. While a primary transfer step is carried out by the intermediate transfer belt 102, the secondary transfer roller 103 is separated from the intermediate transfer belt 102, and when a secondary transfer step is carried out, the secondary transfer roller 103 abuts against the intermediate transfer belt 102.
A transfer cleaner 104 cleans a surface of the intermediate transfer belt 102. The transfer cleaner 104 can also approach and separate from the intermediate transfer belt 102. The intermediate transfer belt 102 is supported by a plurality of rollers, and is driven through a driving roller 105. The transfer cleaner 104 is located at a position opposed to the driving roller 105, and is brought into contact with the intermediate transfer belt 102 under pressure by applying a force to the driving roller 105 through the intermediate transfer belt 102. The transfer cleaner 104 abuts against the intermediate transfer belt 102 after the secondary transfer, and is separated from the intermediate transfer belt 102 before a next primary transfer toner image reaches, and the transfer cleaner 104 removes residual toner (toner which was not transferred) remaining on the intermediate transfer belt 102 (residual toner which was not secondary transferred).
Generally, a blade cleaning is frequently employed for the transfer cleaner 104, and its frictional resistance is high. Thus, if the transfer cleaner 104 approaches and separates from the intermediate transfer belt 102, load variation is generated in a driving roller shaft. A shaft and a gear which are constituent elements of a driving system, a supporting casing of a driving unit are elastic members and thus, elastic deformation is generated by the driving load. Therefore, if the load variation is generated in the driving roller 105 when the transfer cleaner 104 approaches and separates, the elastic deformation amount of the driving system is varied, and the driving roller 105 is displaced in the circumferential direction of the intermediate transfer belt 102.
In recent years, it is required to reduce the apparatus in size and thus, the length of the intermediate transfer belt 102 in the circumferential direction is short, and also when the transfer cleaner 104 approaches and separates, the primary transfer is carried out. For example, if the transfer cleaner 104 is shifted from its separated state to its abutment state during the primary transfer of the fourth color, the driving roller 105 is displaced by a distance δ in a delay direction as shown in the drawings. With this, a YMC image which has already been carried by a transfer belt is deviated in the delay direction by the distance δ with respect to a K image which is the fourth color carried by the photosensitive member drum 101.
FIG. 7(b) shows the actual image recorded in the recording material. FIG. 7(b) shows that in an image formed on a peripheral surface of the photosensitive member drum 101 with the same pitch, the K image which is the fourth color generates color deviation with respect to other colors on the intermediate transfer belt 102 and on the recording material P. FIG. 7(c) is a graph of the color deviation amount of this image. In FIG. 7(c), the horizontal axis shows a sub-scanning direction position (transfer position direction of the recording material), and the vertical axis shows color deviation amounts of the colors based on cyan which is the third color as a reference. It can be found that yellow (Y) that is a first color and magenta (M) that is a second color do not generate color deviation with respect to cyan (C), but black (K) that is a fourth color generates the color deviation in a shrinking direction at a rear end in the image. The color deviation amount is equal to a displacement amount δ on a peripheral surface of the driving roller 105. As one example of an actual numerical value, when a load variation value is 14.7 Ncm for example, color deviation of about 100 μm is generated.
To solve this problem, it is proposed that a direction of an external force (external force acting on the intermediate transfer unit) generated when the transfer cleaner approaches and separates is set to a direction substantially opposite from the rotating/moving direction of the intermediate transfer belt in the primary transfer position, the deviation of the belt in the circumferential direction and the deviation of the unit displacement are offset, and the color deviation is reduced (see Patent Document 1).
[Patent Document 1]
Japanese Patent Application Laid-open No.2002-278204 (paragraph 0081, FIG. 13)